It is given that efficiency ratio =3:1
so time ratio will be 1:3 (since work is same)
Also given that time diff = 32 days. ratio difference = 3-1 =2
2 ratio = 32 days
1 ratio = 16 days.
So A will alone finish it in 16 days and B will finish it in 16*3 = 48 days.
Total work = LCM of 16 and 48 = 48.
Total time = Total work/Total efficiency
ie; 48/4= 12 days.